The present invention relates to a hot-wire type flowmeter which is suitable for use in measuring intake air flow rate in an internal combustion engine.
Flow rate measuring systems are known in which flow rate of a fluid in a main passage is determined by sensing the flow rate in a by-pass passage through which a part of fluid flowing in the main passage flows at the predetermined ratio. This type of system is disclosed, for example, in Japanese Patent Unexamined Publication No. 57-105551, Japanese Utility Model Unexamined Publication No. 56-163668 and U.S. Pat. No. 4,709,581.
In this type of flow rate measuring system having a by-pass passage, the by-pass passage rejoins the intake passage at the narrowest portion of a restriction or orifice provided in the intake passage. Therefore, air is introduced into the by-pass passage, an amount of which corresponds to the air flow in the intake passage, by virtue of the pressure difference generated in the restriction in the intake passage.
Meanwhile, Japanese Unexamined Publication No. 55-48614 discloses a hot-wire type flowmeter in which flow velocity of fluid is measured by making use of the fact that a quantity of heat transferred from a heated electrical resistor wire to the flowing fluid is varied in accordance with the flowing velocity of the fluid.
Electrical resistors for use in hot-wire type flowmeters of the kind described are disclosed, for example, in Japanese Patent Unexamined Publication No. 59-104513, Japanese Patent Unexamined Publication No. 59-151020 and U.S. Pat. No. 4,793,176. These electrical resistors generally have a pipe made of, for example, ceramics, and a platinum wire wound around the pipe and connected at its both ends to lead wires which extend axially through an axial bore in the pipe.
In this type of electrical resistor, both end portions of the platinum wire are beyond both longitudinal end edges of the pipe so as to be connected to the lead wires. The platinum wires tends to slip on these edges and, hence, is liable to be cut, resulting in an inferior productivity
In general, a hot-wire type flowmeter of the type exhibits a higher sensitivity or quicker response to a change in the fluid flow rate or temperature when the heat insulation between the electrical resistor which is to be hot and the associated members of normal temperature is enhanced. Elongating the lead wires on both ends of the electrical resistor is an effective measure for enhancing the above-mentioned heat insulation. As a matter of practice, however, the length of the lead wires is limited because the size of the fluid passage in the flowmeter is limited and because the electrical resistor has to be disposed perpendicular to the flow of fluid in such a restricted passage.
Japanese Patent Unexamined Publication No. 59-104513 discloses an electrical resistor in which lead wires are orthogonally bent to have greater length. This arrangement, however, impairs the mechanical strength of the lead wires.
Japanese Patent Unexamined Publication No. 62-127632 discloses a hot-wire type flowmeter in which an electrical resistor is connected between a pair of lead lines which extend along the fluid passage, in such a manner that the electrical resistor extends across the path of the fluid in the fluid passage. This flowmeter also is impractical because its construction is complicated.